We are utilizing a non-conditional polymerase mutant of B-tropic MuLV to study the mechanism of the reverse transcriptase reaction. This mutant synthesizes partial copies of the natural template in vitro. These products are being characterized in order to determine the nature of the enzymatic blockage to completion of the transcription reaction. At the same time, we are attempting to molecularly clone this mutant in order to completely characterize the genetic change which has led to the structural alteration in the polymerase. We are determining the DNA sequence upstream from transforming gene in two molecularly cloned sarcoma viruses which either do or do not efficiently splice a subgenomic mRNA for the transforming gene. In addition, attention will be given to determining the sequence alterations which are responsible for the production of altered gag gene products by one type of MSV and block production of any gag gene products by the second type of MSV.